This invention relates to a method for densifying powders, and more particularly relates to the densification of powders by mixing them with ice.
Finely divided powders having low bulk densities can present difficult handling problems. For example, operating on such powders in an open room, for example, mixing two powders or transferring a powder from one container to another, can cause losses as well as environmental hazards-in the form of airborne dust.
It is known that handling losses of such powders can be reduced by increasing their bulk densities, for example, by agglomeration. However, where such powders are an unwanted by-product of an industrial operation (herein referred to as "dust") sophisticated agglomerating or pelletizing techniques such as spray drying and disk or pan pelletizing are not appropriate.
In conventional disk or pan pelletizing, both dry powder and liquid are introduced onto a rotating disk or into a pan. In order to effect the desired balling up and densification of the powder, the liquid is conventionally sprayed onto the cascading powder. However, such procedures require the use of expensive equipment and pose difficult material handling problems where toxicity of the dust is a factor. This is because the pelletizing operations are generally carried out in an open environment.
A particularly troublesome disposal problem is presented by toxic furnace dust, such as is generated during the high temperature firing of luminescent materials, often referred to as "phosphors". Such firing promotes the formation of the phosphor composition from a "raw mix" of starting powder materials by solid state diffusion reactions, as well as a multiplicity of intermediate reactions, some of which involve the transport of vapors. Some of these vapors are both volatile and toxic, and are lost from the mix during firing.
For example, the raw mix for calcium halophosphate, a fluorescent lamp phosphor, includes calcium hydrogen orthophosphate, calcium carbonate and calcium fluoride as primary components, as well as ammonium chloride, antimony trioxide, manganous carbonate and cadmium oxide as secondary components.
At temperatures above 500.degree. C., a considerable loss of chlorine and antimony is observed. For this reason, excess halogen is used in the raw mix formulation. The most likely reason for the chlorine and antimony losses is that the hydrogen chloride formed during the dissociation of the ammonium chloride does not react completely with the calcium carbonate, allowing part of it to react with the antimony trioxide, forming antimony trichloride. Antimony trichloride is the most volatile compound that can be formed from the ingredients in the raw mix. It has a melting point of 73.4.degree. C., a boiling point of 220.2.degree. C., and has a considerable vapor pressure, i.e., 3.6 millimeters of mercury at 80.degree. C., and 260 millimeters of mercury at 190.degree. C.
Also taking place at high temperatures are the diffusion reactions that drive the manganese, cadmium and antimony into the crystal lattice of the fluorochlorapatite, to produce the phosphor. The raw mix contains precursors of the above materials in amounts larger than are stoichiometrically required. The excess is vaporized and swept out of the furnace by a small flow of nitrogen gas which continuously passes through the furnace primarily to maintain a slightly reducing atmosphere.
The volatiles that are vented from the furnace are carried away by "sweep air" and ducted to dust collectors. During their passage from furnace to collector, they solidify through condensation and can therefore be removed from the air stream by filtration. Such furnace dust is characteristically light and voluminous, and thus has a very low bulk density. The major components of this furnace dust are cadmium and antimony. Thus, it must be considered a toxic waste which can only be disposed of in an approved manner. Typically, disposal companies charge a fixed fee per container for removal of this toxic waste. Densification of the waste in a simple and straight forward manner could thus represent a significant savings in disposal costs to the manufacturer of the phosphor.
Because the disposal costs are based on a per drum rather than on a weight basis, efforts have been made in the past to pack more dust into each drum. These efforts have taken the form of adding a few inches (about 6 to 8 gallons in a standard drum 42 centimeters in diameter by 69 centimeters high) of water to the drum before placing it under the dust collector. Since the idea is to wet, collapse and densify the dust, a small amount of wetting agent is added to the water.
When the drum has been filled, it is set aside to allow the dust to settle. Then the dust from another drum is added to the extent that settling has taken place.
The above approach is only marginally successful because only the dust in the bottom of the drum becomes wetted down. In addition, the water may soften the wall of the drum or freeze if drums are stored in an unheated area and rupture the wall, eventually leaking out, and carrying toxic material with it. Furthermore, this approach requires handling toxic dust that easily becomes airborne, a health hazard for the operator even if a dust mask is being worn.
Accordingly, it is an object of the invention to provide a simple, inexpensive method of densifying dust.
It is another object of the invention to provide such a method for densifying toxic dust which involves minimum exposure of an operator to the dust.